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Inching Onto Land
1 September 2005 (All day)
The first vertebrates to crawl onto land were a pretty experimental crowd. Some sported feet with seven toes, for example, and strange inner ears. By reconstructing the most famous of these transitional fossils--a creature called Ichthyostega--a team of paleontologists has shown the animal also had an odd gait: an up-down motion, which resembled that of an inchworm and made its movements unlike those of later, more successful amphibians.
Ichthyostega lived about 360 million years ago in what is now Greenland. In previous reconstructions, its entire spine had been extrapolated from a few surviving vertebrae. So the team--Per Ahlberg and Henning Blom of Uppsala University in Sweden, and Jenny Clack of the University of Cambridge, U.K.--pieced together existing and recently discovered material into a new reconstruction of the meter-long animal.
Many paleontologists had assumed that the descendents of fishes kept their side-to-side propulsion when they evolved into four-legged land animals. The new study, however, revealed that Ichthyostega's spine wasn't well-suited to lateral motion. Projections from the top of the vertebrae almost interlocked, constraining the spine to vertical movements in the lumbar region. On land, Ichthyostega probably shuffled along, flexing and extending its lower back to help propel itself. "It must have looked desperately awkward," Ahlberg says, pointing out that it probably didn't have a lot of competition. In the water, Ichthyostega likely flexed its tail to swim.
The vertebral anatomy certainly looks like an adaptation for life on land, says paleontologist Robert Carroll of McGill University, Montreal. But it wasn't a particularly good one, as Ichthyostega was an evolutionary dead end. "It's not a very intelligent design," he says. Other amphibians evolved a different plan--enlarging the central part of the vertebrae--that proved more successful for walking.